The use of polyvalent metal chelates in general, and of iron chelates in particular, is well known for the removal of H2S from sour gas or liquid streams. According to such oxidation-reduction processes hydrogen sulfide gas is removed from a gaseous fluid stream by a continuous catalytic oxidation-reduction system utilizing a ferric iron chelate to catalytically oxidize the hydrogen sulfide and recover elemental sulfur. That is, hydrogen sulfide is converted to elemental sulfur and the ferric chelate is reduced to the ferrous state. The ferrous chelate is subsequently oxidized again to the ferric state to continuously regenerate the ferric iron chelate catalyst.
However, chelating agents on basis of polyamino-poly-acids do not show satisfactory results because they do not operate efficiently at the high pH levels which are required to obtain good absorption of hydrogen sulfide and permit loss of iron as a precipitate of ferric hydroxide and/or ferrous sulfide from the reaction solution. Furthermore, it is generally necessary to add, for example, alkaline metal salts to the system to maintain the operating solution at the desired high pH. This means that there will be a pH value within the operating solution, which exceeds the pH level at which EDTA per Fe is effective. As a result, iron precipitates during the preparation of the solution. Still further, if a base, such as sodium carbonate, is added to a hot iron-EDTA chelate solution, it leads to the formation of foam due to undesired side reactions.
U.S. Pat. No. 4,218,342 discloses a method for preparing an stabilizing iron chelate for use in a scrubbing solution or removing hydrogen sulfide from a gas stream, at a pH of between 5.5 and 13, by combination of two chelate agents such as polyamino polycarbonic acids and of polyhydroxy compounds. During the preparation of the concentrated iron chelate solution, tetrasodium EDTA (Na4EDTA) having a pH of between 11.5 to 13 (as a 5% aqueous solution), is added to a concentrated iron salt solution to form a concentrated iron chelate solution suited for economical transport, storage and make-up in a practical commercial operation. Results show that using this method, some of the iron precipitates, while preparing concentrated solution.
U.S. Pat. Nos. 4,364,871 and 3,767,689, shows that while using methods for preparing aqueous solutions of iron chelates of aminopolycarboxylic acids, the attempts to oxidize the ferrous state to the ferric state by bubbling air through the reaction mixture during the reaction at elevated temperatures apparently decomposes the aminopolycarboxylic acid. That results in the decrease of the amount of free EDTA present in the reaction mixture due to degradation of EDTA during the heat cycle.
In most of the waste streams, the concentration of hydrogen sulfide is relatively low, namely in the order of a few parts per million (ppm) to a few percent. The concentration of carbon dioxide, on the other hand, is relatively higher, in the order of some percent and even up to about 98 percents. If carbon dioxide is present during the operation of the system and treatment of the gas stream, the pH of the solution decreases rapidly in the absorber column due to the absorption of carbon dioxide in the reaction solution. In turn, when the gas stream contacts the ferric iron chelate to oxidize the hydrogen sulfide to elemental sulfur by formation of the ferrous state, the pH value of solution returns slowly to its initial state in the oxidizer zone wherein the ferrous ions are formed and are continuously oxidized to ferric ions, resulting from contact with oxygen.
There is a need in the art for a catalytic composition (and methods of preparation thereof) for removing hydrogen sulfide from a gaseous fluid stream which avoids the above mentioned difficulties.